In some applications lighting elements having high aspect ratios, i.e., having length and width which are much greater than their thickness, are desirable. For example, automotive taillights are preferably designed to be as thin as possible because volume required for such space typically comes at the expense of luggage area. Similarly, the lighting elements used in the displays of laptop computers should be as thin as possible to minimize both size and weight.
One approach to producing such high aspect ratio lighting elements is described in commonly assigned U.S. Pat. No. 4,984,144, filed Feb. 20, 1987, and issued Jan. 8, 1991. According to one embodiment of the invention of that patent, a lighting element uses a light extraction film having a smooth side and a structured side. The structured side has formed thereon isosceles prisms with included angles in the range of 59 to 79 degrees. These included angles are selected because they provide a preferred range of angles in the output light beam.
A limitation of the lighting element shown in FIG. 1 of U.S. Pat. No. 4,984,144 arises from the fact that even nominally collimated beams of light actually will either converge or diverge. Most light sources will produce beams that are slightly convergent. It should be noted, however, that a convergent beam will reach a minimum diameter and then begin to diverge. Effectively, therefore, virtually all typical sources will produce divergent beams. Thus if the smooth surface of the extractor lies in a plane, as shown in that figure, the light beam will strike different portions of the extractor film at different angles. This variation of angularity will be carried into the output beam. This can be a problem in some displays, particularly those utilizing stacked elements. Another related problem arises from the fact that the light intensity falling on the extractor generally will decrease with increasing distance from the light source. This results in nonuniform light output over the optical window.